Various methods and systems have been proposed to recycle refuse and garbage to obtain useful substances, or energy in the form of heat. Usually, the methods use an apparatus which includes an aeration container having a bottom wall to which an aeration apparatus is connected which introduces fresh air to sludge or biological refuse. Municipal refuse is usually classified as trash, which includes essentially non-biodegradable or solid elements, such as old furniture, household discards, metallic objects and the like; and garbage, which usually includes biologically degradable organic substances. Garbage, sludge, and similar substances are often referred to as "biomass". The present invention is directed to recovering heat from biomass upon decomposition of biological refuse which, hereinafter, collectively will be referred to as garbage.
In the known systems, garbage, which already may be decomposed and form sludge, is introduced into an aeration container into which fresh air is introduced, which may result in heating of the garbage-sludge mixture to a temperature of between 50.degree. to 70.degree. C. To sterilize the sludge and garbarge, a temperature of 60.degree. C. over a time of 30' is usually necessary in order to kill enterobacteria, for example salmonellae. This system has the advantage that the heat, necessary for the treatment of the garbage and the decomposition product thereof, which is usually termed sludge, is generated due to the activity of thermophilic microorganisms. The system, however, has the disadvantage that the operation thereof is close to the temperature range at which salmonella bacteria may still remain in viable state. Utilization of the thus generated biological heat for external purposes, for example for heating of buildings or the like, cannot be considered since removal of heat would decrease the temperature of the decomposing substance, and the decomposition temperature may then drop below that at which toxic bacteria are reliably killed.
The energy balance of introducing fresh air into decomposing garbage or sludge is poor; a portion of the air, or even pure oxygen gas introduced into the garbage or sludge, escapes in the form of warmed gas, together with water vapor. The heat loss, thus, is considerable. Due to these losses, substantial decomposition of organic substances is necessary in order to retain the decomposing material, which turns into sludge, at the desired temperature. As the decomposition of the organic substances proceeds, less material is available for subsequent decomposition, so that in the decomposing vessel, less gas will be available.
It has been proposed to use pure oxygen, rather than air, for aeration, since then higher temperatures will occur. The use of pure oxygen is expensive, particularly since the utilization of oxygen is relatively high. Introducing pure oxygen, therefore, is not economically practical.